A unique Middle Ordovician K-bentonite bed succession at Rostanga, S. Sweden

An approximately 8.5 m thick sequence of upper Viruan (upper Middle Ordovician) shales, mudstones, and limestones in an outcrop at Kyrkbacken near Rostanga in W-central Skane contains 19 K-bentonite beds, several of which are as much as 40-67 cm thick. Thirteen of these beds are in the upper part of the Sularp Fm., four in the Skagen Fm., and two questionable beds in the Mossen Fm. Evidence from macrofossils, chitinozoans, and conodonts are used for biostratigraphic age assessment of the K-bentonite succession. Regional comparison of the sequence with those at Kinnekulle (Kullatorp), Koangen, and Tommarp suggests that its total stratigraphic thickness is smaller than those at the two former sites but the thicknesses of several of the Kyrkbacken ash beds are greater than those in similar stratigraphic positions in the other successions. The K-bentonites at Kyrkbacken have a similar clay mineralogy and major and trace element composition as other Ordovician K-bentonites, and these data indicate that the parental magma was of felsic, probably rhyolitic composition. Based on amphibole geobarometry, the magma chamber is interpreted to have been at a depth of 14-20 km. The relatively large number of unusually thick ash beds of Middle Ordovician age makes the easily accessible Kyrkbacken outcrop unique not only in Baltoscandia but, as far as we are aware, also on the entire northern hemisphere, and only one comparable exposure is known in the southern hemisphere, namely in the Precordillera of northern Argentina.